The present invention relates to a semiconductor device and a method of manufacturing the same and, more particularly, a SOI (Silicon On Insulator) type semiconductor device and a method of manufacturing the same.
As the reduction in power consumption of semiconductor integrated circuits and the enhancement in mounting density thereof are furthered, the miniaturization of the individual elements constituting the integrated circuits and the lowering in operating voltages thereof are strongly desired. In the case of a conventional bulk planar type elements, as a result of the miniaturization of the elements and the reduction in channel length thereof, a short-channel effect is actualized; and, in order to prevent it, technical measures such as the enhancement of the impurity density in the substrate, the thinning of the gate insulator, etc. have been taken in accordance with several element size-reduction rules. However, as a matter of fact, as the elements are further and further miniaturized, the existence of some physical limits is encountered; and thus, in order to achieve a further miniaturization, some novel element structures have come to be proposed. As one such novel element structure, there can be pointed out a SOI element which has an insulator under an active region thereof.
Next, typical examples of the structure of an SOI element and the method of the manufacturing the same will be described below. First, FIG. 1 shows a sectional view taken along the direction of the channel length of the SOI element. On a monocrystalline silicon (Si) semiconductor substrate 1, a monocrystalline silicon (Si) active layer 3 is formed through, e.g. a silicon oxide layer (SiO2) 2, and further, a gate electrode 9 is formed through, e.g. a silicon oxide layer (SiO2) 8 which is to be used as a gate insulator. Further, a source region 4-1 and a drain region 4-2 are formed by introducing, by the use of ion implantation method, an impurity of the conductivity type opposite to that of a silicon active layer 4-3 which is to be used as a channel region.
However, the SOI element which has thus been formed is advantageous, in view of improving the element characteristics thereof, in that the film thickness of the active layer can be reduced, but on the other hand, due to the fact that the source and drain diffusion layers or the depletion layer extending from the source and drain diffusion layers reach even the insulator lying under the active layer, it is it is structurally difficult to control the potential in the body region so easily as in the case of a conventional bulk planar type element. As a result, there takes place the phenomenon that the potential in the body region floats during the operation of the element, thus posing problems such as the problem that, during the operation of the element, the threshold voltage of the element changes.
As countermeasures to these problems, attempts have been made to control the potential in the channel region of the thin-film SOI element.
For instance, in Japanese Patent Publication (KOKAI) No. 61-34978, it is proposed to form an electrode, between the isolation region and the buried insulator thereunder, for providing a potential to the channel region from outside. According to this method, however, the isolation insulator is formed in such a manner that the isolation region is previously oxidized into a thin film by selectively controlling the amount thereof, and further, the thicknesses of the contact portion to the channel region and the isolation region are controlled simultaneously and repeatedly again to form the isolation dielectric. Thus, the method has the problem or defect that it is very difficult to control the amount of the SOI layer at the respective manufacturing steps for the reduction in thickness of the SOI layer intended in view of improving the performances, and at the same time, the increase in the necessary area occupied by the element is increased.
As described above, mainly in the case of a conventional thin-film SOI element, there are problems or defects such as the.defect that the manufacturing steps thereof become complicated as compared with the formation of a conventional bulk planar type element, and further, the area occupied by the element is substantially increased.